To address the contradiction between the inadequate ecological functions of traditional seawalls and the safety requirements for coastal protection,physical model tests are conducted on a composite submerged dike-main dike structure (double dike) to explore the optimal design method for eco-friendly seawalls that integrate wave dissipation and habitat creation.By systematically varying parameters such as dike spacing,crest elevation and crest width of the submerged dike,both overtopping discharge and hydrodynamic conditions within the inter-dike area are quantified to synergistically optimize structural safety and ecological performance.The results show that a dike spacing of 0.75 times the incident wavelength achieves the best wave dissipation and flow calming,reducing overtopping discharge by an average of over 50% compared with other schemes.A relative crest width of 2.5 for the submerged dike serves as the optimal threshold,which can basically eliminate overtopping and stabilize the bottom velocity within the inter-dike area at a suitable range of about 0.11 m/s.The installation of the submerged dike reduces inter-dike flow velocity by 40%-60%,resulting in a more uniform and steady flow field,with velocities under all water level conditions well below the 0.4 m/s threshold suitable for benthic organisms.In conclusion,the synergistic design of key parameters enables the double-dike structure to effectively balance wave dissipation and the creation of mild and stable habitats,providing an important basis for the quantitative design of ecological seawalls